Wood pulping process



18, 1964 J. D. ROBERTSON ETAL 3,145,134

WOOD PULPING PROCESS Filed June 4, 1962 INVENTORS: Jun u 5 D. wzoaEarsoN ZMGEQRGE M. New MsoN United States Patent 3,145,134 Wiflrlllf)lllllLPlNG PRGCESS Julius ll). Robertson and George M. Dickinson,Huntsville, S.., assignors to Sonoco Products Company, Harte ville, SAL,a corporation of South Carolina Filed June 4, 1962, Ser. No. 199,743 8Claims. (Cl. 162-4 3) This invention relates to a wood pulping processand more particularly to a process for recovering solubilized woodcomponents in a concentrated form from wood chips in a neutral sulfitesemi-chemical pulping process.

In the pulping of wood chips using a sodium based sulfite process,commonly known as a neutral sulfite semichcmical process, the wood chipsare cooked in a digester in the presence of a cooking liquor to convertcertain Wood components such as the lignins into a solubilized form. Atthe completion of the digestion step, these solubilized wood componentsare then removed and recovered by means of a leaching or washingoperation.

This washing operation is generally carried out by means of suitableapparatus such as vacuum washers in which the solubilized woodcomponents are washed from the digested wood chips using a substantialquantity of dilution water so as to effectively carry out this leachingor washing step. Not only is it generally necessary to wash the chips inseparate apparatus after digestion, but as a result of the large amountof water used in the washing operation, the liquor obtained has a lowsolids content. Therefore, before the liquor can be effectivelyutilized, as is generally accomplished by an extraction or burningoperation, it must be concentrated by evaporation in special apparatussuch as evaporators or the like.

The vacuum washers employed to perform a pulp washing operation and theevaporators used to perform the liquor evaporating operation aregenerally large and expensive. Furthermore, the power required tooperate such equipment, particularly the evaporators where the steamconsumption is substantial, adds considerably to the cost of producingconcentrated black liquor from pulped wood-chips.

Another factor which contributes to the cost of rcovering these woodcomponents during a pulping operation is the inefficiency of the pulpwashing apparatus presently used in semi-chemical pulping. It isgenerally accepted that the formation of the potentially soluble woodcomponents such as lignosulfonates and acetates occurs very early in thepulp digesting process and that approximately one hour of cooking ordigestion in the presence of cook.- ing liquor is sufficient to formthese salts. However, the cooking process is generally extended forconsiderably more than one hour in the process presently used for thepurpose of hydrolytically solubilizing the solid lignosulfonates andacetates and other soluble material and diffusing the resulting solublematerial from the wood chips into solution. It is believed that thepresent day employment of black liquor for said hydrolyticsolubilization and diffusion inhibits the dilfusion of the soluble woodsolids because of the salting out effect of the inorganic salts presentin said black liquor, and accordingly requires additional processingtime for complete diffusion.

Accordingly, a primary object of this invention is to provide a new andnovel method for simultaneously performing the digestion,delignification and liquor concentration operations in a wood pulpingprocess.

Another object of this invention is to provide a new and novel methodfor the recovery of solubilized wood components in a concentrated formfrom wood chips subjected to a pulping process.

Still another object of this invention is to provide a new and novelmethod for the recovery of black liquor in concentrated form from woodchips pulped by a neutral white 3,145,134 Patented Aug. 18, 1964semichemical process which permits the degree of liquor concentration tobe readily determined and which enables the removal of substantially allof the available solubilized wood components in the chips.

A further object of this invention is to provide a new and novel methodfor washing pulped wood chips and for evaporating the black liquorobtained thereby which is highly efiicient so as to require a minimum ofpower consumption and which produces a black liquor in a sufficientlyconcentrated form so that it can be utilized without furtherconcentration.

A still further object of this invention is to provide a new and novelmethod for obtaining concentrated black liquor from wood chips pulped bya neutral sulfite semichemical pulping processwhich eliminates the needfor expensive, bulky, inefiicient and difficult to maintain washing andevaporating apparatus and which materially reduces the time required forobtaining concentrated black liquor in comparison with present daypractices.

This invention further contemplates the provision of a new and novelmethod for simultaneously performing delignification, leaching, andliquor evaporation in a neutral sulfite semi-chemical process forpulping wood chips, which eliminates the need for external dilutionwater, which produces considerable savings in labor and maintenance, andwhich permits the recovery of substantially all of the availablesolubilized wood components from the wood chips in a highly efllcientmanner.

Other objects and advantages of the invention will become apparent fromthe following description taken in connection with the accompanyingdrawings.

In general, the objects of the invention and related objccts areaccomplished by cooking a quantity of wood chips in a digester at apredetermined pressure and temperature in the presence of a cookingliquor by a conventional sulfite pulping process. When the reactionbetween wood components and pulping chemicals which constitutes theinitial phase of the digestion process is completed, free black liquorproduced is conducted into a pressure tank communicating with thedigester and in pressure equilibrium therewith. A head of black liquorin the tank functions as a pressure seal between the tank and digester.Vacuum-compression means are provided for reducing the pressure on thesurface of the liquor in the pressure tank so that the volatilecomponents in the liquor are vaporized to concentrate the liquor,vaporization being limited to the liquor surface as a result of thepressure seal. The vapor is conducted out of the tank and is compressedby the vacuum-compression means. The compressed vapor is then conductedin heat exchange relationship with the liquor in the pressure tank tocondense the vapor and form a condensate. This relatively clearcondensate is then introduced into the digester whereby the condensateflows through the chips in the digester performing the final phase ofthe digestion by hydrolytically solubilizing solid lignosulfonates andother wood components and concomitantly removing by leaching thesesolubilized wood components from the chips. This condensate containingthe solubilized wood components then flows from the digester into thepressure tank in a closed system so that the cycle is continuous.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention, however, both as to its organization and method of operationmay be best understood by reference to the following description takenin conjunction with the accompanying drawing in which:

FIGURE 1 is a diagrammatic View of the preferred form of the apparatusfor practicing the novel method of the invention.

Referring now to the drawing, there is shown one form of apparatussuitable for carrying out the novel method arsena s of the invention.The method of the invention will be described hereinafter with referenceto that pulping process known as a neutral sulfite semi-chemical processin which hardwood chips are cooked in a digester in the presence of asodium based sulfite cooking liquor. As is well known, the activeingredients of this cooking liquor are primarily sodium sulfite andsodium bicarbonate. It should be understood however that the method ofthis invention lends itself to any similar pulping process in which itis desired to separate and recover solubilized wood components in aconcentrated form.

According to present day practices, the wood chips are washed and theliquor resulting from this sodium based sulfite pulping processcontaining dissolved wood solids principally sodium lignosulfonate,sodium acetate, sodium formate, sodium bicarbonate, waxes, tannins,resins, sugars, etc., is recovered and preferably concentrated forsubsequent use. The best practicable concentration of this liquor forfurther use is in the range of about 40% to 45% solids content althoughconcentrations ranging from 25% to 60% solids content may be used. Thisconcentrated liquor can then be burned for its fuel value and its smeltrecovered or further processed for the recovery of certain constituentssuch as acids and the like.

In the novel method of the invention using the apparatus of FIGURE 1,the liquor is recovered in a concentrated form without the need forsubsequent concentration such as by evaporation nor does the novelprocess of the invention require that the chip washing operation beperformed in separate apparatus as presently necessary for removing thesolubilized wood solids.

In the apparatus of FlGURE l, a digester 6, commonly referred to as anupright circulating digester, is provided which is in the form of alarge tank having a side wall 7, a tapered bottom wall 8, a top wall 9having an open end through which a load or charge of wood chips to bepulped can be introduced into the digester. The open end of the digesteris closed by means of a lid 1t and suitable clamping means for clampingthe lid in the closed position are provided.

Communicating with the interior of the digester 6 and extending throughthe digester side wall 7 is a steam conduit 11 having a valve 12therein. A pressure relief valve 13 and a cooking liquor inlet conduit14 having a valve 15 are also provided which communicate with theinterior of the digester 6.

The bottom wall 8 of the digester 6 is provided with an outlet 15 towhich is connected a conduit 17 having a valve 18 therein. The conduit17 is arranged to conduct the pulp from the digester 6, after thesolubilized solids are removed from the wood chips in accordance withthe invention, to a paper machine or the like (not shown) formanufacture into paper in the well known manner.

Also connected to the bottom wall 8 of the digester 6 is a conduit 19communicating with conduits 21, 22 having located therein valves 23, 2respectively. Conduit 21 communicates with a conduit 26 having a valve27 therein and with a conduit 28 extending downwardly into a suitablysupported pressure vessel or tank designated generally by the numeral29. The pressure tank 29 is arranged to be filled to a predeterminedlevel with black liquor obtained from the wood chips pulped in thedigester 6. Conduit 26 communicates with a suitable black liquor storagetank or a burner supply or the like (not shown).

It will be noted that the conduit 28 extends through the top wall 31 ,ofthe tank 29 and throughout substantiallythe entire height of the tank sothat the conduit outlet 32 is adjacent the bottom wall 33 of the tank.As a result of this positioning of the conduit outlet 32, the filling ofthe tank 29 with liquor 34 from the digester 6 to a predetermined levelas indicated by the liquor upper surface 36, there is a substantialheight or head of liquor between the conduit outlet 32 and the liquorupper surface. The upper surface 36 of the liquor 34 introduced into thepressure tank 29 thus defines with the top wall 31 of the tank 29 aspace referred to hereinafter as a vapor zone and designated generallyby the numeral 37.

The vapor zone 37 in the tank 29 communicates by means of a conduit 533connected to the tank top wall 31 with the suction or inlet side ofsuitable vacuum compression means such as a centrifugal compressor 39.While any suitable type of vacuum compression means may be employed,such as a pump or blower, a centrifugal compressor of any well knownconstruction is preferably employed. The outlet or pressure side of thecentrifugal compressor 339 communicates by means of a conduit it? with acoil 41 positioned within the liquor pressure tank 29.

As can be seen, the coil 41 is arranged in spaced relationship with thetop wall 31 of the tank so as to be completely immersed in the liquor34. This fluid and/ or vapor flowing in the coil 4-1 is maintained inheat exchange relationship with the liquor 34. The lower discharge endof the coil communicates by means of a conduit 42 with the conduit 22and a valve 43 is positioned in the conduit 42.

The conduits 22, 42; communicate with a conduit 44 communicating in turnwith the inlet of a pump 45, the outlet of which communicates by meansof conduit 46 with condensate heating means such as a heat exchanger 47.A condensate bleed conduit 43 having a valve 49 is connected to conduitas for removing a portion of the fluid pumped by pump 45, if desired.

The heat exchanger 47 may be of any conventional construction such asthe tube and shell type in which the fluid flowing in its tubes ismaintained in heat exchange relationship with steam or the likesurrounding the tubes. In order to supply steam to the heat exchanger47, a steam supply conduit 51 is provided connected to the heatexchanger as shown and a steam condensate outlet conduit 52 is providedas shown for the removal of the steam condensate from the heatexchanger.

The heat exchanger 47 is arranged to discharge fluid circulated thereininto a conduit 53 connected to the heat exchanger outlet. Conduit 53 isconnected in turn with the top wall 9 of the digester 6 so as tocommunicate with the interior of the digester.

In carrying out the novel method of the invention with the apparatus ofFIGURE 1, hardwood chips are introduced into the digester 6 through thetop opening and the lid 10 is clamped in the closed position as shown.Valves 12, 18 on the digester are opened and all the other valves shownin the drawing, valves 13, 15, 23, 24, 27, 43 and 49 are closed. Steamis then introduced in the top of the digester through the steam conduit11 from a suitable source (not shown) and valve 18 permits the escape ofair and volatile material as well as the drainage of condensateaccumulated in the digester through the conduit 17. This steaming iscontinued until the escape of a steady flow of steam from the conduit 17and/or the attainment of a temperature of approximately C. in thedigester which indicates that the flow of steam has traversed the columnof wood chips and completely filled the digester.

Valves 12, 18 are then closed and valve 15 in the cooking liquor supplyconduit 14 is opened. A predetermined amount of neutral sulfitesemi-chemical cooking liquor, consisting primarily of sodium sulfite andsodium bicarbonate in aqueous solution, is then introduced into thedigester through the supply conduit 14. When the introduction of cookingliquor into the digester 6 has been completed, valve 15 is then closed.

Valve 24 is now opened and steam is supplied to the heat exchanger 47through the steam conduit 51. The pump 45 is started and the cookingliquor in the digester is pumped from the bottom of the digester,through conduit 19 and is conducted through conduits 22, 44,

and 46 in to the heat exchanger 47 wherein the liquor is heated. Theheated liquor then flows out of the heat exchanger 4-7 through conduit53 and is introduced back into the digester 6 at the top.

It should be understood that although heat is introduced into thedigester by heating the liquor as it circulates through the heatexchanger 47, this heat may also be introduced into the digester, ifdesired by the introduction of saturated steam into the charge of chipsand liquor in the digester. This heating and circulation of the liquoris continued until a pressure of about 100-110 p.s.i.g. is attained inthe digester 6.

At this stage in the digestion process, the digester contains a mixtureof volatile gases such as carbon dioxide and steam and as a result thetemperature of the digester charge is significantly less than the gaugepressure would indicate for saturated steam. In order to elevate thetemperature to a value more closely approaching the theoretical value,the valve 13 at the top of the digester is now opened and these volatilegases are permitted to escape. This valve 13 is then closed and thedigestion is continued for about one hour at a pressure of approximately1001l0 p.s.i.g. and at a temperature of about 170 C. At the end of thisperiod of time, the reaction between the sodium sulfite in the cookingliquor and the lignin in the wood chips during which solidlignosulfonates are formed and which constitutes the first phase of thedigestion process is substantially complete.

At this time, valve 24 is closed and valves 23 and 43 are opened. Theliquor from the digester 6 then drains down through conduits 1h, 21 and2% through the conduit outlet 32 into the pressure tank 2% to fill thetank with liquor to approximately the level indicated at 36 so that thecoil 41 is immersed in the liquor as shown. The initial discharge ofliquor from the digester 6 into the presure tank 29 results in a slightpressure drop in the digester but a pressure equilibrium is quicklyestablished between the tank and digester.

Communication between the interior of the digester 6 and the liquor 34in the tank 29 is continuously main tained by means of the conduits 19,21 and 255 as the opening 32 of conduit 28 remains open to permit liquorto pass from the digester into the tank 29, but the pressure equilibriumbetween the tank and digester is also maintained as a result of theinterconnection of the tank 29 and digester 6 in a closed system aspreviously described.

The compressor 39 is now started so that the pressure in the vapor Zone37 above the liquor surface 36 is reduced and evaporation of thevolatile constituents in the liquor 34 is initiated. The head of liquor34 in tank 29 functions as a pressure seal between the tank 29 anddigester 6 so that liquor evaporation is confined to the surface of theliquor in the tank 29 and does not extend to the interior of thedigester 6. The relatively clear vapor is drawn from the tank 29 throughthe conduit 38 as indicated by the arrows and after being compressed inthe compressor 39 is discharged in a compressed state from thecompressor outlet through the conduit id into the coil 41. As a resultof this compression of the vapor, its temperature is raised.

As the compressed vapor circulates through the coil 41 in the directionof the arrows 1, heat is transferred from the compressed vapor to theliquor 34 and the vapor condenses in the coil 41 to form a relativelyclear condensate which is mostly water. This condensate is drawn throughconduits 42, 44 by means of the pump 45 and flows through the heatexchanger 4'7 wherein heat is added to the condensate by the steam fromconduit 51.

The heated condensate then flows through conduit 53 and is introducedinto the top of the digester 6 where it diffuses downward over the chipsby gravity, hydrolytically solubilizing solid lignosulfonates and otherwood components in the second or final stage of the digestion process,concomitantly leaching out these and other mechanically held solubilizedwood components that are present in the chips and further removing woodcom ponents as they are solubilized by hydrolysis. The heating of thecondensate in the heat exchanger 4'7 is carried out at a rate whichinsures that heat loss by radiation and the like is compensated for andthe temperature and pressure within the digester is maintained at apredetermined level so that the digestion process in the digester 6 iscarried out in the shortest possible time and in the most efficientmanner.

As this condensate flows through the digester charge carrying with itsolids from the charge, the chips are washed and hydrolyticsolubilization of solid lignosulfonates and other wood components iseifected in a highly eiiicient manner due to the relatively clearcondensate which is used. This condensate then flows back into the tank20 through conduits 119, 21 and 23 in a closed system constantly addingsolids to the liquor 34%.

Thus, simultaneously with the removal of the wood solids from the chipsin the digester 6 and the transfer of these solids from the digester tothe tank 29, the volatile constituents of the liquor 34 which are mixedwith the solids are continuously removed by vaporization as thecompressor 39 removes the vapor from the vapor zone 37. The liquor 34 isthereby concentrated to the desired degree as the condensate, which isbrought in contact with the wood chips, replaces the solids removed fromthe chips with the chips holding this condensate mechanically forsubsequent removal. The liquor 34 in the tank 29 thus contains a highconcentration of the wood solids which permits the liquor to be usedwithout further evaporation.

If it is desired to increase the degree of concentration of the liquor34 in the tank 29 above that concentration which can be obtained in themanner described above, a portion of the condensate flowing in thesystem can be removed through the outlet conduit 48 by opening the valve&9. By removal of a portion of this condensate, there is less of anexcess of water over that which the chips can absorb and therefore lesswater present to dilute the liquor 34 in the tank 29.

The condensate is circulated through the system as described above untilthe digestion of the chips has proceeded to the desired degree andsufficient solubilized wood components have been removed from the chargein the digester. It has been determined that approximately four hours ofprocessing at a pressure of approximately p.s.i.g. will complete thecook with substantially all of the solubilized wood components containedin the tank 29 and much of the water or condensate held in the chips.

Valves 23 and 43 are now closed, valve 18 is opened and the pulp isblown out of the digester through the conduit 17 for utilization in apaper making machine or the like. Valve 27 is then opened and theconcentrated liquor is blown from the pressure tank 29 to storage or toa burner supply according to the desired end use.

The following is an illustrative but non-limiting example of the processof the invention.

Example Hardwood chips equivalent to about 2500 pounds of bone dry woodsubstance are introduced into the digester 6 and steamed until there isa steady escape of steam through the conduit 1'7 and/ or a temperatureof approximately 100 C. in the digester. Approximately 27.62 cubic feetof neutral sulfite semi-chemical cooking liquor containing about 179.53pounds of sodium sulfite and about 89.77 pounds of sodium bicarbonateboth expressed as equivalents of sodium carbonate are introduced intothe digester. The cooking liquor is circulated and heated until apressure of approximately 100110 p.s.i.g. is reached. The volatile gasesaccumulated in the digester are then blown through valve 13 and thedigestion is con- 'tinued thereafter for about one hour at a pressure ofapproximately 100ll0 p.s.i.g. and at a temperature of approximately 170C. The liquor is then drained from the digester 6 into the pressure tank29 and the compressor 39 is operated to compress vapors from the tankinto condensate and condensate is circulated continuously through thedigester for a period of approximately two to three hours. During thiscirculation of condensate or Washing of the chips, the temperature andpressure in the digester is maintained at the levels referred to above.About 2,000 pounds of bone dry cooked chips comparatively free ofsoluble material remain in the digester 6 and are removed and the blackliquor 34 in the tank 29 has a concentration of about 40% and can beblown from the tank 29 for recovery.

It can be seen that with the novel process of the invention a simple andinexpensive system has been provided by means of which wood chips pulpedby a sulfite pulping process can be simultaneously subjected todigestion and delignification while at the same time the solubilizedwood components can be removed from the chips and can be concentrated toa level which permits utilization of the solids or black liquor withoutfurther treatment and without the introduction of any water other thanthat already present in the system. An outstanding feature of theprocess of the invention is the simultaneous performance of the finalstage of the digestion process wherein the solid lignosulfonates andother soluble wood components are hydrolytically solubilized togetherwith the Washing or leaching of the chips with relatively clearcondensate and concentration of the removed solubilized solids withoutfurther evaporation. In addition, the use of relative- 1y clearcondensate for the hydrolytic solubilization enhances the efficiency ofthis step and eliminates the salting out eflfect on the wood solidsproduced by the organic chemicals in black liquor used in present dayprocesses. The novel method of the invention practiced in this apparatusenables the elimination of separate pulp washing apparatus such asvacuum washers or the like and the elimination of evaporators forconcentrating the black liquor resulting from the pulping process suchas are required in present day pulping processes.

In addition to permitting the use of simple and compact apparatus, thenovel method of the invention is carried out in a highly efficientmanner and with an absolute minimum of power consumption so that notonly is pulp obtained for paper making and the like from which has beenremoved substantially all of the available wood components but theresulting black liquor can be obtained at the desired concentrationwhich is readily controlled in accordance with the unique features ofthe subject invention. The maintenance on the apparatus employed topractice the subject invention is relatively low and the initial cost ofthis apparatus is substantially smaller than presently availableapparatus used for the same purpose. Furthermore, in present daypractices, the amount of water which must be added to such a pulpingprocess for the proper washing of the solubilized wood components fromthe chips is eliminated with the resulting drop in the needed powerconsumption for evaporation.

While there has been described what at present is considered to be thepreferred embodiment of the invention, it will be understood by thoseskilled in the art that van ious changes and modifications may be madetherein without departing from the invention and, therefore, it is theaim of the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

Having thus described the invention, what is claimed is:

1. A pulping process comprising the steps of, reacting wood chips in thepresence of a cooking liquor in a digester by a sulfite pulping processto form solid lignosulfonates and partially complete the digestion ofsaid wood chips, conducting the liquor from said digester to a reservoirin pressure equilibrium therewith whereby the liquor {L3 in saidreservoir is isolated from the chips in said digester, evaporating thevolatile components of the liquor in said reservoir to form a vapor,condensing said vapor to form a condensate, introducing said condensateinto said digester in intimate contact with said wood chips to completesaid digestion by hydrolytically solubilizing said solid lignosulfonatesand other components in said wood chips while simultaneously removing byleaching said solubilized wood components from said wood chips to form aliquor and adding said liquor to the liquor in said reservoir in aclosed system to thereby transfer the solubilized material from saidwood chips to said reservoir.

2. A process in accordance with claim 1 including the step of removing aportion of said condensate from said system to further concentrate theliquor in said reservoir.

3. The pulping process comprising the steps of, reacting wood chips inthe presence of a cooking liquor in a digester by a sulfite pulpingprocess to form solid lignosulfonates and partially complete thedigestion of said wood chips, conducting the liquor from said digesterto a vessel in pressure equilibrium therewith, isolating the liquor insaid vessel from the chips in said digester, evaporating the volatilecomponents of the liquor in said vessel to form a vapor, condensing saidvapor to form a condensate, introducing said condensate into saiddigester in intimate contact with said wood chips to complete saiddigestion by hydrolytically solubilizing said solid lignosulfonates andother components in said Wood chips while simultaneously removing byleaching said solubilized wood components from said wood chips to formliquor and adding said formed liquor to the liquor in said vessel in aclosed system to thereby transfer the solubilized material from saidwood chips to said vessel.

4. A pulping process comprising the steps of, reacting Wood chips in thepresence of a cooking liquor in a digester by a sulfite pulping processto form solid lignosulfonates and partially complete the digestion ofsaid wood chips, conducting the liquor from said digester to a tank inpressure equilibrium therewi h, isolating the surface of the liquor insaid tank from the interior of said digester, reducing the pressure onthe surface of the liquor in said tank to vaporize the volatilecomponents in said liquor, removing said vapor from said tank,compressing said vapor, conducting said compressed vapor in heatexchange relationship With the liquor in said tank to heat said liquorand to condense said compressed vapor, introducing said condensate intosaid digester in intimate contact with said wood chips to complete saiddigestion by hydrolytically solubilizing said solid lignosulfonates andother components in said wood chips while simultaneously removing byleaching said solubilized wood components from said wood chips to formliquor and adding said formed liquor to the liquor in said tank in aclosed system to thereby transfer the solubilized material from saidwood chips to said tank.

5. The pulping process comprising the steps of, reacting wood chips inthe presence or" a cooking liquor in a digester by a sulfite pulpingprocess to form solid lignosulfonates and partially complete thedigestion of said Wood chips, conducting said liquor from said digesterto a tank in pressure equilibrium therewith to form a head of liquor insaid tank, utilizing said head of liquor in said tank to form a pressureseal between the interior of said digester and the upper surface of theliquor in said tank,

reducing the pressure on the surface of the liquor in said tank tovaporize the volatile components in said liquor, removing said vaporfrom said tank, compressing said vapor, conducting said compressed vaporin heat exchange relationship with the liquor in said tank to heat saidliquor and to condense said compressed vapor, introducing saidcondensate into said digester in intimate contact with said wood chipsto complete said digestion by hydrolytically solubilizing said solidlignosulfonates and other components in said wood chips whilesimultaneously removing by leaching said solubilized wood componentsfrom said 9 wood chips to form a liquor and adding said formed liquor tothe liquor in said tank in a closed system to thereby transfer thesolubilized material from said wood chips to said tank.

6. A pulping process comprising the steps of, reacting wood chips in thepresence of a cooking liquor in a digester by a sulfite pulping processto form solid lignosulfonates and partially complete the digestion ofsaid wood chips, conducting the liquor from said digester to a tankthrough a conduit having an outlet positioned a suitable distance belowthe surface of the liquor in said tank to define a head of liquor,forming a pressure seal with said head of liquor to isolate the uppersurface of the liquor in said tank from the interior of said digester,reducing the pressure on the surface of the liquor in said tank tovaporize the volatile components in said liquor, removing said vaporfrom said tank, compressing said vapor, conducting said compressed vaporin heat exchange relationship with the liquor in said tank to heat saidliquor and to condense said compressed vapor, introducing saidcondensate into said digester in intimate contact with said wood chipsto complete said digestion by hydrolytically solubilizing said solidlignosulfonates and other components in said wood chips whilesimultaneously removing by leaching said solubilzed wood components fromsaid wood chips to form a liquor and adding said formed liquor to theliquor in said tank in a closed system to thereby transfer thesolubilized material from said wood chips to said tank.

7. A pulping process comprising the steps of, reacting wood chips in thepresence of a cooking liquor in a digester by a sulfiite pulping processto form solid lignosulfonates and partially complete the digestion ofsaid wood chips, conducting the liquor from said digester to a tankthrough a conduit having an outlet positioned a suitable distance belowthe surface of the liquor in said tank to define a head of liquor,forming a pressure seal with said head of liquor to isolate the uppersurface of the liquor in said tank from the interior of said digester,providing vacuum-compression means having a suction side incommunication with said liquor upper surface, reducing the pressure onsaid liquor surface with said vacuum-compression means to vaporize thevolatile components in said liquor, removing said vapor from saidvessel, compressing said vapor in said vacuum-compression means,conducting said compressed vapor from said vacuum-compression means inheat exchange relationship with the liquor in said tank to heat saidliquor and to condense said compression vapor, introducing saidcondensate into said digester in intimate contact with said wood chipsto complete said digestion by hydrolytically solubilizing said solidlignosulfonates and other components in said wood chips whilesimultaneously removing by leaching said solubilized wood componentsfrom said chips to form a liquor and adding said formed liquor to theliquor in said tank in a closed system to thereby transfer thesolubilized material from said wood chips to said tank.

8. A pulping process comprising the steps of, reacting wood chips in thepresence of a cooking liquor in a digester by a sulfite pulping process,recirculating said cooking liquor through a heat exchanger to heat saidcooking liquor during said reacting step to form solid lignosulfonatesand partially complete the digestion of said wood chips, conducting theliquor from said digester to a tank having a bottom wall through aconduit having an outlet adjacent said tank bottom wall thereby defininga head of liquor extending from the upper surface of said liquor to saidconduit outlet, forming a pressure seal with said head of liquor toisolate the upper surface of the liquor in said tank from the interiorof said digester, providing vacuum-compression means having a suctionside and a pressure side with said suction side in communication withsaid liquor surface, subjecting the surface of the liquor in said tankto a reduced pressure at the suction side of said vacuum-compressionmeans to vaporize the volatile components in said liquor, removing thevapor from said vessel with said vacuum-compression means, compressingsaid vapor in said vacuum-compression means, conducting said compressedvapor from the pressure side of said vacuum-compression means through acoil immersed in the liquor in said tank in heat exchange relationshiptherewith to heat said liquor and to condense said compressed vapor,pumping the condensate from said coil through said heat exchanger,heating said condensate to a predetermined temperature in said heatexchanger, introducing said heated condensate into said digester inintimate contact with said wood chips to complete said digestion byhydrolytically solubilizing said solid lignosulfonates and othercomponents in said Wood chips while simultaneously removing by leachingsaid solubilized wood components from said wood chips to form a liquorand adding said formed liquor to the liquor in said tank in a closedsystem to thereby transfer the solubilized material from said wood chipsto said tank.

References Cited in the file of this patent UNITED STATES PATENTS1,669,234 Dunbar May 8, 1928 1,793,264 Tschuwikowsky Feb. 17, 19311,852,011 Hatch Apr. 5, 1932 2,041,597 Dunbar a May 19, 1936 2,329,727Rosenblad Sept. 21, 1943

1. A PULPING PROCESS COMPRISING THE STEPS OF REACTING WOOD CHIPS IN THEPRESENCE OF A COOLING LIQUOR IN A DIGESTER BY A SULFITE PULPING PROCESSTO FORM SOLID LIGNOSULFONATES AND PARTIALLY COMPLETE THE DIGESTION OFSAID WOOD CHIPS, CONDUCTING THE LIQUOR FROM SAID DIGESTER TO A RESERVOIRIN PRESSURE EQUILIBRIUM THEREWITH WHEREBY THE LIQUOR IN SAID RESERVOIRIS ISOLATED FROM THE CHIPS IN SAID DIGESTER, EVAPORTING THE VOLATILECOMPONENTS OF THE LIUQOR IN SAID RESERVOIR TO FORM A VAPOR, CONDENSINGSAID VAPOR TO FORM A CONDENSATE, INTRODUCING SAID CONDENSATE INTO SAIDDIGESTER IN INTIMATE CONTACT WITH SAID WOOD CHIPS TO COMPLETE SAIDDIGESTION BY HYDROLYTICALLY SOLUBILIZING SAID